For increasing the efficiency of feeding papers in a duplex mode, the current office machine such as a copying device, a printing device or a scanning device is usually equipped with a duplex document feeding device. By the duplex document feeding device, a stack of papers may be automatically fed into the office machine in the duplex mode in order to be scanned or printed.
FIG. 1 is a schematic cross-sectional view illustrating a conventional duplex document feeding device. As shown in FIG. 1, the conventional duplex document feeding device 10 comprises a first paper transfer channel 11, a second paper transfer channel 12, a paper exit 13, a paper ejecting mechanism 14, a solenoid valve 17, and plural transferring rollers 18. The paper ejecting mechanism 14 comprises an ejecting drive roller 15 and an ejecting idler roller 16. In addition, the ejecting drive roller 15 is contacted with the ejecting idler roller 16.
Please refer to FIG. 1 again. The paper ejecting mechanism 14 is located at the paper exit 13 for ejecting the paper from the duplex document feeding device 10 or introducing the paper into the duplex document feeding device 10. The solenoid valve 17 is connected with the ejecting idler roller 16 of the paper ejecting mechanism 14 for driving movement of the ejecting idler roller 16.
Moreover, the transferring rollers 18 are disposed in the second paper transfer channel 12 for transferring the paper within the first paper transfer channel 11 or the second paper transfer channel 12. The first paper transfer channel 11 and the second paper transfer channel 12 are disposed within the duplex document feeding device 10 for allowing the paper to go through.
A process of feeding papers in a duplex mode will be illustrated in more details as follows. After the paper is fed into the duplex document feeding device 10, the paper is transferred through the first paper transfer channel 11 and ejected from the paper exit 13. Then, the paper is introduced into the second paper transfer channel 12 by the paper ejecting mechanism 14. Then, the paper is ejected from the paper exit 13 by the transferring rollers 18.
In a case that the length of the paper transferred through the second paper transfer channel 12 is too longer, the front end of the paper is possibly overlapped with the portion of the paper which is just fed into the second paper transfer channel 12. Under this circumstance, the front end of the paper fails to be smoothly transmitted to the paper exit 13. For solving this drawback, after the paper is introduced into the second paper transfer channel 12 by the paper ejecting mechanism 14 and transmitted to the transferring rollers 18 to assure that the paper can be continuously moved, the ejecting idler roller 16 is shifted by the solenoid valve 17 of the duplex document feeding device 10. Meanwhile, the ejecting idler roller 16 is moved away from the ejecting drive roller 15, and thus the front of the paper can be smoothly transferred through the paper exit 13.
From the above discussions, the use of the solenoid valve to move the ejecting idler roller away from the ejecting drive roller may result in a gap between the ejecting drive roller and the ejecting idler roller. Due to the gap, the front end of the paper is no longer overlapped with the portion of the paper just fed into the second paper transfer channel, so that the front of the paper can be smoothly transferred through the paper exit. However, there are still some drawbacks. For example, since the unit price of the solenoid valve is high, the use of the solenoid valve will undoubtedly increase the fabricating cost of the duplex document feeding device.